Aluminum is conventionally manufactured by the electrolysis of alumina (AL.sub.2 O.sub.3) which is reduced to metallic aluminum. The reduction is carried out in a bath of fused cryolite (3NaF.cndot.ALF.sub.3) that is maintained within a pot having a carbonaceous potlining, with the pot serving as the cathode in an electrolysis reaction at about 1600.degree.F. to about 1800.degree.F. During use over extended periods of time, the carbonaceous potlinings of such pots gradually deteriorate as the materials in the pot penetrate into the carbonaceous lining material, resulting in ultimate leakage of the bath and of metallic aluminum.
Hundreds of thousands of tons of spent potlinings are generated each year in the production of aluminum. Spent carbonaceous potlinings, as a result of their use, deterioration and penetration by bath materials, contain significant quantities of fluorides as well as aluminum, alkali metals, nitrides, and some cyanides. Because spent potlinings comprise certain materials, such as some fluorides and cyanides, which are categorized as hazardous wastes, the generation, storage and disposal of the potlinings have an environmental impact. Because a significant portion of spent potlinings is carbonaceous material, burning or combustion of such spent potlinings is one way of potentially meeting the disposal problems. Indeed there are a number of U.S. patents which are directed to this general concept; for example, U.S. Pat. Nos. 4,444,740 to Snodgrass et al. and 4,053,375 to Roberts et al. are concerned with the alleviation of a potential environmental problem with the output of such a combustor and with the recovery of some reusable material, such as cryolite or fluorine, from the potlinings. More recently, U.S. Pat. No. 4,763,585 to Rickman et al. discloses the burning of spent potlinings that have been reduced to particulate size under fluidized bed conditions where agglomeration of the bed material is positively prevented.
Although combustion of spent potlinings can provide an efficient means for destroying carbonaceous potlinings and cyanides contained therein, it requires relatively high temperatures, and oftentimes economic usage cannot be made of the high temperature effluent stream that is created. For example, stable combustion of spent potlinings has been found to require temperatures which exceed 1400.degree.F. Maintenance of these temperatures may be undesirable and uneconomical, and other treatment methods have been sought after.